Optically amplified transmission systems offer important advantages over current optical fiber transmission systems using electrooptic regenerators which include higher bit rate capability and multichannel transmission via wavelength division multiplexing. The optical fiber amplifiers previously have been configured in two stages for the purpose of achieving both high gain and high signal output power. An example of a single-stage optical amplifier is disclosed in U.S. Pat. No. 5,050,949, issued Sep. 24, 1991, and assigned to Applicants' assignee. An example of a prior art two-stage optical fiber amplifier design is disclosed in the article "High Gain Two-Stage Amplification with Erbimum-doped Fibre Amplification" by Masuda and Takada in Electronics Lett. 26, 661 (1990).
In these prior art teachings, minimizing the pump power of the optical fiber amplifier typically is not of concern because of the availability of continuous and adequate power. Minimizing pump power and at the same time maintaining sufficiently high gain and signal output is critical, however, to several important applications of optical fiber amplifiers. Battery-powered short-haul underwater telecommunications systems and data communications systems internal to automotive vehicles are two such instances.
In attempting to adapt a conventional two-stage optical fiber amplifier to a system with a low power requirement, simply lowering the pump power substantially sacrifices performance. Thus, prior art two-stage optical fiber amplifiers and single-stage amplifiers as well do not suffice where a substantially reduced pump power requirement must be met.